The explosive increase in Internet accessibility and use offers unique opportunities for providing health information to adolescents via the Internet. The purpose of this evaluation study was to explore the opinions of adolescents regarding an Internet-based health monitoring instrument and its individually tailored electronic feedback so that recommendations for improvement can be provided. A self-administered evaluation questionnaire was completed by 672 adolescents aged 12-18 years the Dutch equivalent of the 8th and 10th grade of secondary education. Semistructured group interviews were conducted with 53 adolescents to obtain more in-depth information regarding the monitoring questionnaire and the individually tailored feedback. All the respondents preferred the electronic monitoring questionnaire to a paper and pencil questionnaire and the individually tailored feedback was appraised rather favorably. However, 28% of the respondents claimed that the information was not new to them and 39% indicated that the information failed to provide them with additional insight into their behavior. In order to increase the number of adolescents reached, we recommend (i) embedding monitoring and feedback in school curriculum, (ii) providing immediate feedback and (iii) adapting tailored messages to educational levels and age. Although several improvements can and should be made, we conclude that the Internet-based monitoring questionnaire and the individually tailored feedback were appreciated by adolescents and are therefore a promising method for engaging adolescents in health promotion.